Touch input systems detect touch events, such as a touch from a user's finger, a stylus, or some other device. Touch regions of the touch input systems are often transparent for use with an information display of a computer or other electronic system. Other touch input systems are opaque touch sensors, such as touch or track pads. Touchscreens and other touch input systems are used in a variety of applications, such as information kiosks, retail points of sale, order entry systems (e.g., restaurants), industrial process control applications, interactive exhibits, mobile phones and other personal electronic devices, and video games.
Some touch input systems use acoustic signals to detect touch events. Certain types of acoustic touchscreens, also known as ultrasonic touchscreens, detect touch with high transparency and high resolution, while providing a durable touch surface. Of particular commercial interest are ultrasonic touchscreens using surface acoustic waves (SAW).
SAW touchscreens often have a glass overlay on which transmitting and receiving piezoelectric transducers are mounted. A controller sends an electrical signal to the transmitting transducer, which converts the signal into ultrasonic waves on the surface of the glass. These waves are directed across the touchscreen by an array of reflectors. Reflectors on the opposite side direct the waves to the receiving transducer, which reconverts the waves into an electrical signal. The process is repeated for each axis. A touch absorbs a portion of the waves traveling across the touch region on the surface. The received signals for X and Y are compared to stored digital maps, the change is recognized, and a coordinate is calculated from the center of the “shadow” or touch signal in the form of an attenuation dip in the received signal. For single touch operation, effects that spread out or disperse in time the touch signal are of only secondary importance as they have little effect on the touch coordinate extracted from touch signal centers.
Problems arise for many touchscreens when two or more touch events occur simultaneously. The multiple touches cause two or more X and two or more Y attenuation locations. When the separation between a pair of touches becomes small, such as at the end of a two-finger zoom-out or pinch gesture or at the beginning of a two-finger zoom-in gesture, the shadows or attenuation dips may blur together confounding touch coordinate data.